Bricks, mortar and a very thin wall
The skin is usually described in three layers: the epidermis on top, the dermis beneath it, and the fatty subcutis below that. The barrier we talk about in skincare lives at the very top of the epidermis, in a structure called the stratum corneum. It is made of corneocytes — flattened cells that have lost their nucleus and filled up with the tough protein keratin — stacked in roughly 15 to 20 layers.
Between those cells sits a highly organised lipid matrix. By weight it is roughly half ceramides, about a quarter cholesterol and around 10 to 20 percent free fatty acids. These lipids arrange themselves in layered sheets, a little like the pages of a closed book, which is what makes the barrier so hard for water to escape through. Researchers including Peter Elias at the University of California, San Francisco, popularised the "bricks and mortar" model in the 1980s, and it remains a useful mental picture.
Natural moisturising factor
Inside the corneocytes is a mix of water-attracting molecules known as natural moisturising factor, or NMF. It includes amino acids, urea, lactic acid and a compound called PCA, mostly derived from the breakdown of a protein called filaggrin. NMF keeps the cells plump and flexible. People with certain filaggrin gene variations — common in atopic eczema — produce less of it, which helps explain why their skin dries and cracks so easily.
The acid mantle
Healthy skin surface pH sits around 4.7 to 5.5. That mild acidity is not decoration: the enzymes that assemble ceramides work best in it, and it discourages the growth of some unwanted bacteria. Very alkaline cleansers, including many traditional bar soaps with a pH near 9 to 10, can raise surface pH temporarily. Healthy skin recovers within hours; already-irritated skin recovers more slowly.
